1. Field
The present invention relates to a method for calibration of a grid plate. Furthermore, the invention relates to a lithographic apparatus configured for the calibration method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the known lithographic apparatus high accuracy position measurement systems are used to determine the position of movable objects such as wafer stage and reticle stage. Two known systems which are used are interferometer systems and encoder-type measurement systems. In the encoder type measurement systems use is made of one or more one-dimensional gratings which may be combined in multi-dimensional grids. These gratings or grids may be arranged on a one-dimensional or multi-dimensional grid plate. The grid plate may be mounted on a substantially stationary part of the lithographic apparatus. One or more encoder-type sensors capable of measuring the position of the sensor with respect to the grid plate in one or more degrees of freedom are mounted on the movable object. In alternative embodiments the grid plate may be mounted on the movable object and the sensor may be mounted on a substantially stationary location.
For certain applications, in particular for use in a lithographic apparatus, the fabrication accuracy of the grating is not sufficient to obtain the desired accuracy needed for these applications. For instance, the fabrication accuracy of a grating may be on a micron level, while a stage position measurement system requires a sub-nanometer accuracy.
To take these manufacturing errors into account a so-called metrology map may be used in which the errors of the grating are incorporated. These errors are obtained during calibration of the grating and consequently incorporated into the metrology map. During actual use of the measurement system the values of the metrology map are used to correct for the errors in grating.
A known calibration method for determining the errors and making a metrology map is the so-called fishbone-like technique. In this technique, multiple measurements are made in an area of a calibration location and the results of the measurements are averaged to obtain the calibration value of the calibration location. To obtain a desired accuracy for a certain calibration location a minimum area in which the multiple measurements are made, is required. For instance, when an accuracy of 0.1 nm has to be obtained a grid spacing, i.e. the distance between adjacent calibration locations, of about 2-5 mm is required. However, with increasing requirements on the accuracy of the measurements, the calibration grid spacing may not be sufficient to obtain the desired accuracy during actual position measurement. For instance to obtain a 0.1 nm accuracy during actual position measurement, the same grating of the above example has to be calibrated with a grid spacing of about 0.4 mm. Thus, a drawback of the state of the art calibration methods is that the calibration grid spacing becomes too large when the required accuracy of a encoder measurement system is increased.